Hobby / Craft stores sell 8x10" sheets of smooth foam. You can build up layers under the reel seat as needed.

BUT WAIT, THERES MORE !

Buy a few extra sheets, it's the same foam thats used in a lot of fly boxes. Use it to replace ratty,rusty,ripped foam in your fly boxes. Glue it down with rubber cement, or for permanent placement use contact cement.
As a side benefit, I like to give my sons and fishing buddies my home tied flies. Use pieces of foam to turn pocket sized mint tins ( like Altoids) into fly boxes to hand out the flies in. It's fun to see someone pull one of those boxes out of a vest.

__________________You can always tell a fly fisherman. You just can't tell him much.